This invention relates to an improved anti-corrosive pigment comprising an iron oxide-calcium oxide sintered composite and to an improved anti-corrosive aqueous paint containing the pigment.
As well known, anti-corrosive pigments are widely used at present in a shipbuilding industry and a building industry for preventing the generation of rusts in iron material.
Anti-corrosive paints generally involve those consisting of a paint forming matrix, composed of vehicle (resin), solvent and extender pigment, coloring pigment, drier, surface active agent and the like, blended with anti-corrosive pigment. The conventional anti-corrosion paints well known so far include those composed of the foregoing paint forming matrix blended with an anti-corrosive pigment such as a lead compound, for example, red lead oxide, basic lead sulfate, a chromate series compound, for example, zinc chromate, strontium chromate or the like, etc.
In recent years, however, environmental pollution caused by noxious metals such as lead, chromium and the like have come to the fore and the production and the use of the red lead oxide, basic lead sulfate, zinc chromate, strontium chromate and the like essentially consisting of lead, chromium or the like are gradually restricted.
Accordingly, provision of non-pollutant anti-corrosive pigment as a substituent for the above lead compounds, chromate compounds and the likes has been highly demanded in the industries concerned. In response to the foregoing demand of the industry, the inventors of the present application have made a study long since and established a novel technique of using as the anti-corrosive pigment a powder comprising iron oxide-calcium oxide sintered composite which is prepared under the specific production conditions (refer to Japanese Pat. No. 855,733 and U.S. Pat. No. 3,904,421).
The process of producing the anti-corrosive pigment developed by the inventors and the effects obtained therefrom are to be described hereinafter.
Referring at first to the process for the production, it comprises the steps of mixing 40-70% by weight, calculated as iron oxide, of an iron compound and 30-60% by weight, calculated as calcium oxide, of a calcium compound, calcinating the mixture at a temperature between 700-1150.degree. C. to produce an anti-corrosive pigment of iron oxide-calcium oxide sintered composite containing more than 5% 2CaO.Fe.sub.2 O.sub.3.
The anti-corrosive effect of the anti-corrosive pigment obtained by the process above described owes to the presence of 2CaO.Fe.sub.2 O.sub.3 therein and the mechanism is as follows.
2CaO.Fe.sub.2 O.sub.3 reacts with water into 3CaO.Fe.sub.2 O.sub.3. nH.sub.2 O and, at the same time, turns basic through the dissolved calcium ions. Moreover, a part of the calcium ions turns to calcium hydroxide with the action of water and another part of the calcium ions turns to carbonate products with the action of gaseous carbon dioxide. In the course of the conversion into 3CaO.2Fe.sub.2 O.sub.3.nH.sub.2 O, it is hydrated with water as water of crystallization. Consequently, 2CaO.Fe.sub.2 O.sub.3, when deposited on the surface of a steel plate or the like, keeps the surface of the plate basic by its basicity. Moreover, it forms a tight coating film on the surface of the steel plate due to the formation of carbonate products and, in addition, reduces the moistures present on the surface of the steel plate by hydrating them as water of crystallization. The above phenomena provide a strong anti-corrosive effect.
However, an anti-corrosive aqueous paint containing the above iron oxide-calcium oxide sintered composite has been unfortunately encountered a problem that gellation occurs in the paint during storage thereof and makes it impossible to use in some cases.
The reason why the gellation occurs will be explained below. It is generally known that a pigment in a paint, when it is chemically active (for example basic), reacts with a resin in the paint-forming matrix to form a metal soap. Those substances tend to form such metal soaps include metals such as Pb, Mn, Cu, Al, Zn, Cr and Ca. The above anti-corrosive pigment developed by the inventors shows its anti-corrosive effect by the reaction of 2CaO.Fe.sub.2 O.sub.3 contained therein with water as foregoings. Accordingly, in an aqueous paint containing the above anit-corrosive pigment, some 2CaO.Fe.sub.2 O.sub.3 is dissolved out into water which is employed as a solvent to form calcium ions, whereby rapid reaction between the dissolved calcium ions and the resin is taken place to produce a metal soap causing the gellation to occur in the paint. Therefore the above iron oxide-calcium oxide sintered composite, if used as it is as the anti-corrosive pigment to produce the anti-corrosive aqueous paint, the resulted paint has an extremely poor storage stability and, in an extreme case, causes gellation already in the course of the paint production. Since the gellation in the paint is caused through the above mechanism, gellation can be prevented by minimizing the amount of water which is used as the solvent, whereby the calcium component dissolved in water can be decreased in proportion to the reduction of water in the paint. Since the pain thus produced is a high viscosity paint or a high resin solid content (NV), it can be used by adding an appropriate amount of water to dilute the paint in a viscosity suited to caoting application. However, in this case the paint left after the use gells as well during storage.